That's a great question! I've just filmed a video to answer it, but it will be a couple of days before it's up.
In the mean time,
The stitches you're supposed to knit look like V's. The stitches you're supposed to purl look like dashes. You can watch the video on doing ribbing (Basic Techniques/More page), which talks about how to recognize a purl and knit stitch.
KnittingHelp Queen Bee
"There is a vitality, a life force, an energy, a quickening that is translated through you into action, and because there is only one of you in all of time, this expression is unique. And if you block it, it will never exist through any other medium and it will be lost. The world will not have it. It is not your business to determine how good it is nor how valuable nor how it compares with other expressions. It is your business to keep it yours clearly and directly, to keep the channel open. You do not even have to believe in yourself or your work. You have to keep yourself open and aware to the urges that motivate you. Keep the channel open."
— Martha Graham